Principles of Plant Genetics and Breeding

Applications The scheme has been used in crops such
as maize and sunflower with reported genetic gains of
2.17% for the population per se, and 4.90% for the population
hybrids.
Procedure: cycle 0
Season 1 Plant population A as females (detassel) in
an isolated block and population B as males
in field 1. Plant population B as females and
population A as males in field 2. The upper
ears in each field are open-pollinated, while
the lower ears are protected and pollinated
manually. The result is that the upper ear is
an interpopulation half-sib family, while the
lower ear is an intrapopulation half-sib family.
Season 2 Evaluate 100–200 A × B and B × A half sibs in
replicated trials. Select the best half sibs from
both sets of crosses.
Season 3 Plant the remnant seed of the lower ears
(selfed by hand pollination) that correspond
to the best A × B half sibs in ear to rows as
females (detassel). The males are the bulk
remnant half-sib seed from population B corresponding
to the best B × A crosses. They
are randomly mated. The open-pollinated
seed in populations A and B are harvested to
initiate the next cycle.
Genetic issues An advantage of this method is that additive
genetic variance of full-sib families is twice that of the
half-sib families. The expected genetic gain is given by:
∆G = iσ A 2 /2σPFS
where σ PFS = phenotypic standard deviation of the fullsib
families.
Advantages
1 As compared to the half-sib method, one-half of the
families are evaluated in each cycle because the evaluation
of each full sib reflects the worth of two parental
plants, one from each population.
2 Superior S 0 × S 0 crosses may be advanced in further
generations and evaluated as S 1 × S 1 , S 2 × S 2 ,..., S n
× S n to allow the breeder to simultaneously develop
hybrids while improving the populations.
Optimizing gain from selection in
population improvement
The goal of the breeder is to make systematic progress
in the mean expression of the trait of interest from one
BREEDING CROSS-POLLINATED SPECIES 327
cycle to the next. Achieving progressive gains in yield
depends on several factors.
1 Genetic variance. As previously indicated, it is critical
to increase additive genetic variance per cycle.
Additive genetic variance can be increased through
increasing diversity in the entries used in population
improvement.
2 Selection intensity. The rate of gain with selection
is increased when selection intensity is increased. The
number of individuals selected for recombination in
each cycle should be limited to the best performers.
3 Generations per cycle. The breeder’s choice of the
breeding system to use in a breeding project is
influenced by how rapidly each cycle of selection can
be completed. When possible, using 2–3 generations
per year can increase yield gains. Multiple generations
per year is achieved by using off-season nurseries
(winter nurseries), or planting in the dry season using
irrigation.
4 Field plot technique. Breeders select in the field, often
handling large numbers of plants. Heterogeneity
in the field should be managed by using proper
experimental designs to reduce random variation.
Whenever possible, uniform fields should be selected
for field evaluations. The cultural conditions should
be optimized as much as possible (proper fertilization,
irrigation, disease and pest control, weed control,
etc.). This practice will reduce variation between
replications. Other factors to consider are plot sizes,
number of plants per plot, number of replications per
trial, and number of locations. Implemented properly,
these factors reduce random variations that complicate
experimental results.
Development of synthetic cultivars
Synthetic cultivars versus germplasm composites
There are two basic types of open-pollinated populations
of crops – those produced by population improvement,
and synthetics. As previously discussed,
population improvement methods can be categorized
into two – those that depend on purely phenotypic
selection (mass selection), and those that involve selection
with progeny testing. A synthetic cultivar may be
defined as an advanced generation of cross-fertilized
(random mating in all combinations) seed mixture of
parents that may be strains, clones, or hybrids. The parents
are selected based on GCA. The primary distinction
between the basic types of population mentioned in